Friday, September 6, 2013

MilliCrabs and petrichor

In my work as a science editor, I run across unfamiliar terminology. Sometimes it's baffling, and sometimes it's charming. Here are some recent examples of the charming kind.

Magnetic local time: OK, this one requires a bit of an intro. Universal Time (UT) is a timekeeping system in which the day arbitrarily begins when it's midnight in Greenwich, England (at longitude 0°). Each location has its own local time (UT plus or minus so many hours) based on the difference between its longitude and that of Greenwich. The longitude lines used in this kind of timekeeping are part of a latitude–longitude grid that converges at Earth's physical north pole. Similarly, you can make a grid of lines that converge on Earth's magnetic north pole. Then the magnetic local time is midnight when your line of longitude faces away from the sun and noon when it faces the sun.

Petrichor: This lovely word describes the scent of rain on dry earth, which is an absolutely heavenly thing if you live in a dry climate. Certain plant oils (notably those of the creosote bush) are absorbed by the soil and stone of the desert floor and released into the air when it rains, along with another compound from the wet soil, producing the heady aroma. The word is derived from the Greek word for stone (petros) and the Greek word ichor, which was what the gods of Greek mythology supposedly had instead of blood. (The words petroleum and petrify can also be traced back to petros.) This scent lies behind the title of a beautiful book that Gary Paul Nabhan wrote about the Sonoran desert and the Tohono O'Odham people who live there: The Desert Smells Like Rain: A Naturalist in O'odham Country (public library).

Crab and milliCrab: The Crab is a measure of the X-ray output of an astronomical source at a particular wavelength, pegged to that of the Crab nebula, a supernova remnant in the constellation of Taurus. Because the Crab nebula is a powerful X-ray source, milliCrabs (thousandths of a Crab) are often used.
Auroral chorus and dawn chorus: These are both radio waves that are naturally produced when energetic electrons are injected into in Earth's magnetosphere; they can be converted into somewhat eerie singing sounds (chirps, hoots, whistles). The dawn chorus is most likely to occur around sunrise, and the auroral chorus is associated with aurorae. Curiously, both names can be traced back to Aurora, the Roman goddess of the dawn. The northern and southern lights (aurora borealis and aurora australis) were named for their beautiful subtle colors, which resemble those of sunrise.

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